1. Field of the Invention
The present invention relates to a process for distillative separation of pure (meth)acrylic acid from mixtures which comprise (meth)acrylic acid and dimers and oligomers of (meth)acrylic acid and are essentially free from aldehydes and other components whose boiling point is lower than that of (meth)acrylic acid.
2. Description of the Background
(Meth)acrylic acid is used as an abbreviated written form and denotes acrylic acid or methacrylic acid. (Meth)acrylic acid, either per se or in the form of its esters, is particularly important in the preparation of polymers for a very wide variety of applications, for example as an adhesive, and especially in the liquid aggregate state has a high propensity to polymerize. Safe storage of essentially pure liquid (meth)acrylic acid is possible even at low temperatures only with the addition of polymerization inhibitor.
(Meth)acrylic acid is obtainable inter alia by catalytic gas phase oxidation of alkanes, alkanols, alkenes or alkenals having 3 or 4 C atoms. (Meth)acrylic acid is obtainable with particular advantage, for example, by catalytic gas phase oxidation of propene, acrolein, tert-butanol, isobutene, isobutane, isobutyraldehyde or methacrolein.
When considering possible starting compounds, however, one should not ignore those from which the actual C3/C4 starting compound is formed only during the gas phase oxidation, as an intermediate. An example which may be mentioned is the methyl ether of tert-butanol.
In the reaction these starting gases, generally diluted with inert gases such as nitrogen, CO2, saturated hydrocarbons and/or steam, are passed in the form of a mixture with oxygen and at elevated temperatures (usually from 200 to 400xc2x0 C.) at atmospheric or superatmospheric pressure over transition metal (containing Mo, V, W and/or Fe, for example) mixed-oxide catalysts and are converted by oxidation to (meth)acrylic acid (cf. e.g. DE-A 44 05 059, EP-A 253 409, EP-A 92 097, DE-A 44 31 949).
Because of the numerous parallel and follow-on reactions that occur in the course of the catalytic gas phase oxidation, and because of the inert diluting gases that are used, the catalytic gas phase oxidation produces not pure (meth)acrylic acid but instead a reaction mixture which essentially comprises (meth)acrylic acid, the inert dilution gases and byproducts and from which it is necessary to separate the (meth)acrylic acid.
(Meth)acrylic acid is usually separated from the reaction mixture by way of extractive and distillative separation techniques. In such techniques, the (meth)acrylic acid formed is typically first of all absorbed from the gas phase oxidation reaction mixture into a suitable absorbent. Subsequent distillative separation of the absorbate customarily gives a crude (meth)acrylic acid which, on traversing further distillative separation stages, frequently produces a pure (meth)acrylic acid (cf. e.g. DE-A 44 36 243, DE-C 21 36 396, DE-A 43 08 087, EP-A 297 445, EP-A 117 146, EP-B 102 642, GB-B 1 346 737 and DE-C 22 07 184).
A feature of all of these rectificative separation techniques, irrespective of whether the (meth)acrylic acid is separated off at the top or at the bottom and even when polymerization inhibitors are used, is that after a relatively short time during the rectificative separation a deposit is formed within the rectification devices, as a result of which rectification, which is normally carried out continuously, must be interrupted at intervals to allow removal of the deposit formed.
In recent times extensive investigations have been carried out into how to reduce the problem of the formation of deposits during the distillative separation of (meth)acrylic acid from the reaction mixtures or product mixtures obtained in the preparation of (meth)acrylic acid.
For instance, DE 195 01 326.3 describes a method of rectificative is separation of (meth)acrylic acid from a mixture comprising (meth)acrylic acid as principal constituent and lower aldehydes as secondary constituents in a rectification column which consists of a stripping section and a rectifier section, where the starting mixture comprising the (meth)acrylic acid that is to be separated by rectification is not supplied directly to the rectification column but instead is first of all passed into a heated dwell vessel, connected on the vapor side with the rectifier section of the rectification column, in which vessel the starting mixture is held at boiling, and instead of the starting mixture per se the bottom liquid from the residence vessel is supplied to the rectification column. In this method, the mixture is processed to generally liquid mixtures with a (meth)acrylic acid content of from 5 to 25% by weight, as are generally obtained following the addition of an absorbent to the reaction mixture obtained from the catalytic gas phase oxidation, and after subsequent desorption, as the outgoing flow from the desorption column. In addition to (meth)acrylic acid, these mixtures still comprise large amounts of absorbent and lower aldehydes as secondary components.
A more recent process for the continuous distillative separation of liquid mixtures whose principal constituent is (meth)acrylic acid is described in DE 195 39 295.7, which is also directed mainly toward the problem of reducing the formation of deposits during rectification. According to this application, the distillative separation of a mixture which in general has a content of (meth)acrylic acid of xe2x89xa795% by weight (crude (meth)acrylic acid) is conducted in a distillation device which has a distillation still, a condenser and a connection between distillation still and condenser, and to which the liquid mixture that is to be separated is supplied continuously, wherein a fraction of the distillation still liquid is withdrawn in superheated form and is passed back under pressure release into the distillation device in the course of this process.
All of the processes described in the prior art have the common feature that the bottom product of the distillations or rectifications, which contains up to 99% by weight (meth)acrylic acid as the target substance, is not likewise subjected to distillative workup. The resulting bottom product, which contains a relatively high proportion of (meth)acrylic acid in the form of oligomers, has to date been used as a component in a coupled production process for butyl (meth)acrylate. Coupled production of this kind is, however, not always possible or economic.
It is an object of the present invention, accordingly, to provide a process which is able to isolate pure (meth)acrylic acid, as simply as possible and with a high yield, from the (meth)acrylic acid which is present in the bottom product. A further object, to be achieved with the aid of this process, is to produce not only as much pure (meth)acrylic acid as possible but at the same time to produce a correspondingly small amount of residue requiring landfill or incineration.
We have found, in the course of the investigations leading to this invention, that this object is achieved, surprisingly, by a simple distillation process.
The present invention accordingly provides a process for distillative separation of pure (meth)acrylic acid from mixtures which comprise (meth)acrylic acid and dimers and oligomers of (meth)acrylic acid and are essentially free from aldehydes and from components whose boiling point is lower than that of (meth)acrylic acid, such as water and acetic acid, for example, using a distillation apparatus which has a thin-film evaporator, a condenser, and a connection which contains a baffle device and links the thin-film evaporator and the condenser.